Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

AM radio - please help!

  1. Jul 18, 2004 #1
    AM radio -- please help!


    Here is some background info. I don't know if it is all necessary to find a solution but here it is anyway:

    I am currently living in State College, PA. I would like to be able to listen to an AM radio station broadcasting from Philadelphia, PA. Philadelphia is about 200 miles from State College. State College is located in the mountains and is approximately 1200 feet above sea level. Philadelphia is at sea level.
    The radio station I am interested in receiving is 610 AM.

    What would I need to do to receive this radio station?
    Do AM waves travel through mountains or such obstacles that may be in the way? Would I need to buy an AM radio receiver? If so, what should I look for (any suggestions on what radio to buy)?

    I cannot think of any other questions...I'd really appreciate any help that can be provided! Thanks so much!
  2. jcsd
  3. Jul 19, 2004 #2
    Does the Radio station not stream over the internet?
  4. Jul 19, 2004 #3
    AM radio not available online

    No, unfortunately they do not stream online...any suggestions?
  5. Jul 19, 2004 #4


    User Avatar
    Science Advisor
    Homework Helper

  6. Jul 19, 2004 #5


    User Avatar
    Science Advisor
    Gold Member

    As opposed to what? Stealing or building one?

    You can buy some higher end receivers geared towards short wave listening but they have good performance on AM too. Just make sure you get one that requires an outside antenna. Anything else is going to have no better reception than the average radio. Now for the outside antenna. You need a fairly long wire antenna to receive 610 KHz. Build a 'dipole' antenna. It is a half wave electrical length. This is about 767 feet long for 610. It is split in the center by an insulator. The feedline (antenna cable) connects to the center. Ideally any antenna should be at least a half wave length above the ground. I don't see this happening with you. The best way to support an antenna like this is from the center and ends.
  7. Jul 19, 2004 #6
    Ummm...that's very interesting.

    Averagesupernova, assuming that was a serious answer (was it?), what's the reason behind the half-wavelength antenna size you suggested.

    Also, how did you arrive at 767 feet? I thought you might be using [tex]c=f\lambda[/tex] but that gives me
    [tex]\frac{\lambda}{2} = \frac{c}{2f} = \frac{2.998e8 * 3.281}{2*610,000} = 806 \text{ft.}[/tex]
    so should I be calculating it some other way?
  8. Aug 17, 2004 #7
    Speed of light in metal vs speed of light in a vacuum

    Electromagnetic waves propagate slower through metal than they do through the air or through vacuum. If his answer is 767, and your answer is 806, then the conversion factor is 95.16 percent (IOW, he may have been assuming light travels ~95% as fast in metal as it does in a vacuum).
    Last edited: Aug 17, 2004
  9. Aug 19, 2004 #8


    User Avatar
    Science Advisor
    Gold Member

    Hmmmm. Sorry I hadn't checked back in on this sooner. Yes, it IS a serious answer. How I arrived at the length of 767 is from a somewhat standard formula used for a half wave dipole. It is: 468/frequency in Mhz = length in feet for a half wave dipole. The formula is not perfect. Surrounding objects can affect the tuning of the antenna. For what you want to use it for it is close enough. But, if you want to get REALLY technical you need to get some test equipment and adjust the length of the antenna based on measurements with the test equipment.

    You also asked what the reason for the half wave type antenna was. ANY decent antenna needs to be resonant. Some antennas are not half wave physically but they still form a resonant circuit. The reason I suggested what I did was because it is a simple design. It is indeed large, but simple with few components.

    Now I know what your next question is. Can I get by with a shortened antenna? The answer is yes, but at reduced performance. What you can do is wind coils and insert them midway down each shortened leg of the dipole. This adds inductance so to counter it and keep the resonant frequency where you want it, the length is shortened.

    If you need to know any more I would suggest reading a book. Start with amateur radio.
  10. Jul 15, 2005 #9


    User Avatar
    Science Advisor
    Homework Helper

    By your discussion it sounds like you may have done some hamming. Nice job recommending a dipole for 610KHz. I agree, he may hear a better signal if this antenna was mounted at least 1/2 [tex]\lambda[/tex] above the ground. As you point out, this is impractical. The best thing would be to string it as high as he can.

    An additional point worth mentioning; to improve gain (signal strength), if he knows the compass direction to the station in Philly (approximately southeast from State College), he should mount the dipole perpendicular to that direction. Since zeus00 does mention the station is 200 miles away, the best time to hear it, would be after dark.

    If he is ready to try something with more gain, perhaps a log-periodic array pointed E-SE, is in order. :biggrin:
    Back on a serious note, he could improve his dipole by adding director and reflector elements. Even adding a single reflector (0.1 [tex]\lambda[/tex] behind and parallel to the dipole), can improve gain up to 6dB.

    glossary: [itex]\lambda[/itex] = wavelength
  11. Jul 20, 2005 #10
    I suggest a phased array ferrite antenna for high gain with a high Q LC
    front end tuned to the station you want. This is experimental of course and
    would require a large plot of land to space the individual antennas, but hey-

    Why not?
  12. Jul 27, 2005 #11


    User Avatar
    Science Advisor
    Homework Helper

    Granted, a dipole with an additional element(s) is starting to push the limit of practicality. A phased array for 610KHz (vertical antenna farm) would be over the top. I don't see it as feasible for a student to build, to receive his/her favorite AM broadcast station. (Each vertical element would be about 384 ft high, require suitable guying, using a minimum of two elements at a spacing of 1/2[itex]\lambda[/itex] or 767ft. In other words it would like the photo on this page.) :uhh:
  13. Aug 2, 2005 #12

    No, you misunderstood (though it's probably over the top anyway.)

    I'm suggesting small ferrite loop antennas like the ones in any common
    AM handheld receiever, but yes spaced as you depict. This is NOT outside the
    realm of expertise of a dedicated amateur radio operator.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook